Spectrophotometers measure reflected light intensity at selected wavelengths. They are widely used in graphic arts and similar applications to ensure uniform color hue and density of printed material, paints, dyed fabric, and other applications where ink, paint, dye, or other coloring material is applied to paper, plastic, fabric, walls or other surfaces or materials. Standardized measurement of light reflectance at different wavelengths allows printing pressmen and other equipment operators to adjust a printing press, color copier, or other colorant application device or system so as to obtain uniform, consistent color hue and density of applied colorant material. In some systems the measurements of light reflectance are used to adjust printing and other color application systems semi-automatically or automatically.
To assist in making standardized measurements, printed material typically has color regions of different hues to act as reference color samples. Typically these are printed in the material margins. Color printing device or printing press may print color sheets that contain color patches or stripes. The color regions can be measured by a spectrophotometer or other device for color hue and density measurement. Numerous spectrophotometers and systems exist for measuring reflected light color and brightness from these patches or from other parts of the printed material. However existing spectrophotometers and systems have limitations.
Hand-held, single-measurement spectrophotometers exist that are accurate, easy-to-use, and relatively inexpensive. However they only make a single measurement at a time, and typically a printed color sheet of paper must be removed from a sheet fed press, or the printing press stopped for a web fed press, to make measurements. After the operator makes a change to the printing press to optimize color hue and density, the press may need to be re-started and another measurement made to determine if the printed output is now acceptable. This is a tedious and time-consuming process which leads to higher costs for printing companies.
Spectrophotometer scanners exist with an additional “look-ahead” sensor or sensor array that aligns the spectrophotometer with color patches. These sensors add cost and complexity to the system because of the added sensor and its support hardware (and possibly software). The look-ahead sensor must be carefully aligned with the spectrophotometer, both mechanically and in time, to ensure that spectrophotometer alignment tracks sensor alignment, and periodically checked to ensure that alignment remains valid. Again this adds cost to the printing process for printing companies.
Spectrophotometer scanners further exist that move the measuring device in two axes, facilitating the measurement of stationary samples. These devices are more complex and they solve the more complex case of a target that may be angularly misaligned with the scanner. In solving that, the system must make multiple scans across the length of the measured sample, and compute a best multi axis trajectory for future sample measurements. If the media is moving, the measured artifacts typically remain aligned with the direction of motion, so motion across the media is not necessary, and all steps required to estimate angular error are not providing additional value.
In-line spectrophotometer systems also exist that use a video camera as a sensor to align the in-line spectrophotometer with the color regions so as to measure the color regions printed on the sheet. In some cases, these systems automatically adjust the printing press or other device to optimize color hue and density. Again, these systems are complex, very expensive, and typically require periodic maintenance and calibration for continued proper function.
Automated systems exist that use a color video camera to take a real-time picture of the printed material, including color patches, to determine printed color hue and density. In some cases, these systems automatically adjust the printing press or other device to optimize color hue and density. However, these systems are complex, very expensive, and typically require periodic maintenance and calibration for continued proper function. They are also limited by the performance of the color video camera and its illumination system for consistent and uniform color measurement.
A semi-automated spectrophotometer can be mounted on a printing press and that measures color patch hue and density in real-time as printed material passes underneath the spectrophotometer. The system uses proprietary pattern recognition software to detect the patches passing longitudinally under the spectrophotometer, and automatically detects and measures each color patch as it goes by. However, the lateral position of the spectrophotometer must be adjusted to align with the position of the color patches on the printed material. This can be very difficult for smaller patch sizes.
What is needed is a system and method that improves the performance of spectrophotometer devices for real-time measurement of color hue and density by automating the process of laterally centering the spectrophotometer over the reference color regions in the measurement system.